Article pubs.acs.org/IC
Dihydrogen Activation by Cobaloximes with Various Axial Ligands Gang Li, Deven P. Estes, Jack R. Norton,* Serge Ruccolo, Aaron Sattler, and Wesley Sattler Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027, United States S Supporting Information *
ABSTRACT: We have investigated the effect of axial ligands on the ability of cobaloximes to catalyze the generation of transferable hydrogen atoms from hydrogen gas and have learned that the active catalyst contains one and only one axial ligand. We have, for example, shown that Co(dmgBF2)2 coordinates only one Ph3P and that the addition of additional Ph3P (beyond 1 equiv) to solvated Co(dmgBF2)2 does not affect its catalytic turnover for H• transfer from H2.
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were taken on either a Bruker 300, 400, or 500 MHz spectrometer. IR spectra were taken with a PerkinElmer Spectrum 2000 FT-IR spectrometer. UV−vis spectra were recorded on a Hewlett-Packard spectrophotometer. X-ray diffraction data were collected on a Bruker Apex II diffractometer. Crystal data, data collection, and refinement parameters are summarized in the Supporting Information. The structures were solved using direct methods and standard difference map techniques and were refined by full matrix least-squares procedures on F2 with SHELXTL (version 6.1).8 Benzene and THF were distilled from K/benzophenone ketyl and stored over 3 Å molecular sieves. All other liquids, including benzene-d6, were dried by distillation from CaH2 and then deoxygenated by three freeze−pump− thaw cycles and stored under an argon atmosphere. Materials. Co(dmgBF2)2(H2O)2,9 Co(dmgBF2)2(MeCN)2,3e and Co(dmgBF2)2(MeOH)29 were synthesized by known procedures. Tris(p-tert-butylphenyl)methyl radical was prepared by the process described in the literature.10 Co(dmgBF2)2(THF)2. Diethyl ether (150 mL, O2 free) was added to a flask containing Co(OAc)2·4H2O (2.0 g, 8 mmol) and dmgH2 (1.9 g, 16 mmol), followed by freshly distilled BF3·Et2O (10 mL, an excess). The mixture was stirred for 6 h under argon. The resulting solid was filtered under argon, washed with ice−cold THF (3 × 5 mL, O2 free), and dried under vacuum. A red solid product was obtained (3.2 g, 4 mmol, 50% yield). IR (ATR): 2968, 2880, 1615 (CN), 1438, 1385, 1210, 1164, 1092, 1046, 994, 935, 880, 817, 627, 603, 578, 502, 468 cm−1, which includes peaks like those that have been attributed to coordinated THF.11 UV−vis (C6H6, 7.6 × 10−5 M, 1 cm quartz cell): 445 nm [ε = 3.3(2) × 103 M−1 cm−1]. Kinetic Measurements. Caution! Pressure reactions should be handled with care behind a blast shield. Otherwise serious injury may result! All experiments were performed at 300 MHz. In a typical experiment, a stock solution of trityl radical in C6D6 was prepared with an internal standard (hexamethylcyclotrisiloxane), along with a stock solution of cobaloxime in C6D6. Appropriate amounts of the two stock solutions were mixed in a thick-walled J-Young tube in the glovebox, and the tube was sealed and removed from the box. After three freeze−pump−thaw cycles to remove the argon from the solution,
INTRODUCTION The use of cobaloximes to catalyze chain transfer in radical polymerizations was suggested by Rempel in 1984,1 and the effectiveness of these complexes at catalyzing chain transfer under H2 was shown in a 2006 patent.2 Cobaloximes and their derivatives have also been shown to be catalysts for the production of hydrogen from protons and electrons, a reaction which is of interest to solar fuels production.3 cobaloxime catalysts
2H+ + 2e− ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ H 2
(1)
We have recently examined cobaloxime/hydrogen reactions in reverse and have found that Co(dmgBF2)2(H2O)2 under H2 can serve as a source of H•.4 For example, Co(dmgBF2)2(H2O)2 catalyzes the hydrogenation of the monomeric trityl radical5 in reaction 2. The rate of reaction 2 is independent of [Ar3C•]
(Ar = p-tBuC6H4), but first-order in H2 and second-order in cobaloxime, suggesting that the slow step is the activation of H2 by the cobalt catalyst. It seems likely that H• transfer arises from a Co(III)−H hydride intermediate; such intermediates appear to be involved in the catalysis of hydrogen evolution. The ability of cobaloximes to catalyze the production of H2 from protons is substantially affected by the nature of their axial ligands.6 (The effect of trans ligands L on the reacitivity of ligands X in cobaloximes and related compounds has been reviewed.7) We have thus examined the effect of axial ligands on the ability of cobaloximes to catalyze hydrogenations like the one in reaction 2.
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EXPERIMENTAL SECTION Received: August 18, 2014 Published: September 18, 2014
All manipulations were performed under an argon atmosphere using standard Schlenk or inert atmosphere box techniques. NMR spectra © 2014 American Chemical Society
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hydrogen gas was added until the desired pressure was reached. The tube was allowed to thaw, and its contents were mixed before it was placed in the probe of the NMR (whose controller was already set to the desired temperature, 295 K). A methanol standard was used for temperature calibration. The H• transfer product, tris(p-tertbutylphenyl)methane, was identified by comparing its 1H NMR spectrum with that of an authentic sample. The integration of the product methine peak relative to that of the internal standard was recorded as a function of time. Sixteen pulses were used for every kinetic point, with 1 s between pulses. The molar concentration of H2 in benzene was calculated by Henry’s law.12
Table 1. Selected Bond Lengths of Co(dmgBF2)2L2 L
M−L (Å)
M−N (Å)
NC (Å)
C−C (Å)
ref
CH3CN MeOH THF
2.260 2.264 2.324
1.878 1.877 1.890
1.297 1.270 1.299
1.479 1.473 1.483
this work 9 this work
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RESULTS AND DISCUSSION Structure Discussion. In both solution and solid state, cobaloximes exist as planar macrocyclic complexes with axial ligands L, Co(dmgBF2)2Ln (n = 1, 2) (Figure 1). When L is a
Figure 1. General structure of Co(dmgBF2)2L2.
coordinating solvent molecule, the L = 2 form Co(dmgBF2)2L2 dominates and the axial ligands are easily exchanged. Co(dmgBF2)2(MeOH)2 and its X-ray structure have been reported,9 and Co(dmgBF2)2(MeCN)2 has been characterized by UV−vis spectroscopy in CH3CN solution.3e We have prepared crystals of the L = THF and the L = CH3CN adducts and have confirmed by X-ray diffraction that they have the structures below (Figure 2a, 2b), with the two ligands L coordinated axially as in the MeOH complex. Selected bond distances for the MeOH, THF, and CH3CN complexes are shown in Table 1. EPR studies have shown that stronger σ-donor ligands (e.g., Ph3P, pyridine) form cobaloxime complexes with only a single axial ligand (L = 1).13 In toluene/CH2Cl2 Co(dmgBF2)2(py)2 is only formed in the presence of a 10-fold excess of py,13 although EPR showed that Co(dmgBF2)2(py)2 is also formed
Figure 3. Co(dmgBF2)2(THF)2-mediated formation of Ar3CH from Ar 3 C• under 2.4 atm of H 2 in C 6 D 6 at 295 K (eq 3): [Co(dmgBF2)2(THF)2] = 3.80 × 10−4 M and [Ar3C•]0 = 1.02 × 10−2 M.
when a frozen matrix of Co(dmgBF2)2 is allowed to warm to room temperature.14 Kinetics of Hydrogenation. We have examined the kinetics of the hydrogenation of Ar3C• radicals in benzene with Co(dmgBF2)2(THF)2 as a catalyst (eq 3). The rate is once
Figure 2. (a, left) Molecular structure of Co(dmgBF2)2(CH3CN)2 (50% ellipsoids, hydrogen atoms omitted for clarity). (b, right) Molecular structure of Co(dmgBF2)2(THF)2 (50% ellipsoids; hydrogen atoms are omitted for clarity, the BF2 groups and the THF ligand are disordered, and only the main contribution to the disorder is shown for clarity). 10744
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Figure 5. Rate d[Ar3CH]/dt of eq 3 vs [THF]. All rates were measured at 295 K in C6D6.
Scheme 1. Proposed Mechanism for the Hydrogenation Reaction
⎛ K[CoTot ] ⎞2 d[Ar3CH] = k1⎜ ⎟ [H 2] ⎝ K + [L] ⎠ dt
(8)
The data with added THF fit the rate law in eq 8 if k1 is 1.02(1) × 103 M−2 s−1 and K is 1.26(6) M. (The amount of THF added far exceeds the amount that arises from dissociation.) The results of similar experiments with MeOH in C6D6 fit the same rate law if k1 is 346(7) M−2 s−1 and K is 0.128(6) M, respectively (see the Supporting Information). The larger dissociation constant K for L = THF does correlate with the longer M−O distance in the THF adduct. In neither case is there any evidence for dissociation of the second ligand. It is clear that the nature and concentration of free axial ligand L affect both K and k1 (see Table 2). It is, unfortunately, impractical to determine these constants for other ligands from the kinetics of trityl hydrogenation. With L = CH3CN, the rate of the reaction with H2 is negligible; with L = H2O the low solubility of water in benzene makes the determination of K impractical. In order to compare k1 for THF and MeOH with the rate constant for a stronger σ donor, we examined the catalysis of Ar3C• hydrogenation by Co(dmgBF2)2(PPh3). We were unable to isolate that complex, but we were able to confirm that only a 1:1 adduct is formed in CH2Cl2 solutionthe same conclusion reached in toluene/CH2Cl2 by the EPR study cited above.13 The addition of PPh3 to a solution of Co(dmgBF2)2(H2O)2 changes its UV−vis spectrum (Figure 6), but there are no further changes after 1 equiv of PPh3 has been added. A Job
Figure 4. (a) Rate d[Ar 3 CH]/dt of the hydrogenation of Ar 3C• vs [H2]. (b) Rate d[Ar 3CH]/dt of the hydrogenation of Ar 3C• vs [Co(dmgBF2) 2(THF)2]. All rates were measured at 295 K in C6D 6.
again independent of [Ar3C•] (Figure 3), first-order (Figure 4a) in [H2], and second-order (Figure 4b) in [CoTot], where [CoTot] is the concentration of cobaloxime initially added. The rate law is thus the one given in eq 4. d[Ar3CH] = k[H 2][CoTot]2 (4) dt The data in Figure 4 give a third-order rate constant k (defined in eq 4) of 1.04(6) × 103 M−2 s−1. Experiments in C6D6 with different concentrations of added THF (Figure 5) show that the reaction is inhibited by that ligand, suggesting a dissociative preequilibrium (equilibrium constant K) like that shown in Scheme 1. Subsequent H 2 activation and H• transfer steps, like those found in our previous study of the diaquo complex, 4 complete a plausible mechanism. 10745
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Table 2. Equilibrium Constant K and Rate Constants k and k1 for Dihydrogen Activation by Cobaloximes with Different Axial Ligands and M−L Bond Distancesa ligand k1(M−2 s−1) k (M−2 s−1)a K (M) M−L (Å)
THF 1.02(1) × 103 1.04(6) × 103 1.26(6) 2.324
MeOH 346(7) 343(7) 0.128(6) 2.264
PPh3b 37(5) 37(5) − −
H2O − 106(3) − −
MeCN − k1(MeOH) > k1(PPh3). The slowness of Co(dmgBF2)2(PPh3) may be partly the result of its “bowl-shaped” structure, as calculated by Niklas et al.,13 which will make more difficult a reaction (like eq 6) that is secondorder in cobalt. (There appears to be steric hindrance to the approach of P(n-Bu)3 to a five-coordinate alkylcobaloxime.20) Overall it is apparent that H2 is best activated by five-coordinate cobalt, with a single axial ligand: Co(dmgBF2)2(THF), Co(dmgBF2)2(MeOH), or Co(dmgBF2)2(PPh3). The H2/cobaloxime reaction is the reverse of the bimolecular mechanism that has been suggested3a for the evolution of H2 from hydrides HCo(dmgBF2)2L.
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ASSOCIATED CONTENT
S Supporting Information *
Full preparative, analytical, and crystallographic details. Crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by DOE Grant DE-FG0297ER14807. We are grateful to Boulder Scientific and OFS Fitel for additional support. D.P.E. is supported by the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF), made possible in part by the American Recovery and Reinvestment Act of 2009 and administered by ORISE-ORAU under Contract DE-AC05-06OR23100.
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REFERENCES
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